The microelectronic technique, the core of which is a large scale integrated circuit, is rapidly developed in a principle of “one generation of technique for one generation of apparatuses and one generation of devices”. With appearance of nano-devices, there is provided new demands for the micro fabrication and micro- and nano-detection technique. An optical projection photolithography technique utilizes an optical projection theory to transfer a pattern from a mask to a silicon wafer in a high resolution by a projection objective lens. The optical projection photolithography technique has a high resolution for a contact-type photolithography and may avoid damaging and contaminating a mask. As the microfabrication technique with a widest application fields, rapidly updated technique and a strong viability, the optical projection photolithography technique is a core to drive the development of the microelectronic techniques and at present it is employed in most large scale integrated circuit
In an optical projection photolithography machine, resolution and focal depth for the projection objective lens are key parameters affecting a projection and exposure system. The focal depth is drastically reduced with enhancement of the resolution of the photolithography machine. Meanwhile, in order to improve production efficiency of the projection photolithography machine, exposure areas of the silicon wafer are gradually increased. Thus, there is a new demand for the accuracy for adjusting the focal plane of the system by the photolithography machine with a high accuracy.
In a conventional projection and exposure photolithography machine, since the focal depth is long, the photolithograph machines mostly employ a technique for detecting the focus by CCD or PSD sensor. By imaging with a slot, a defocusing distance of a silicon wafer may be calculated by computing change of displacement of the slot in the detector. These two detecting methods have a simple measuring system and are easy to be operated; but they have a lower accuracy of measurement and can't meet the requirements of the photolithography on the detection of the focal plane in a higher accuracy.
With the enhancement of resolution of the photolithography system and the enlargement of the exposure visual field, the measuring system for adjusting leveling and the focal plane may further utilizes a multipoint measurement in which the height values at multiple points are measured to calculate amounts of inclination of the silicon, which is mainly used by a Japanese company of Nikon. An emergence light of a light source passes through an array of slots, is reflected by a surface of the silicon wafer and is imaged on the detector. When the silicon wafer is in an ideal state, the imaged light spot is located at the center of four-quadrant detector so that the light intensities in the four quadrants are identical. The detection system utilizes the array of slots to cover the whole exposure visual filed by scanning measurements. However, it is necessary to further optimize the algorithm for detection to improve the accuracy of measurement.
With the development of grating detection in the last century, a detection technique based on moiré fringe of a grating is applied to measurements for detecting focal plane in a projection photolithography system. If the relative positions of the grating and the detector is constant, the signal of the moiré fringe changes due to the change of position of the silicon wafer. Thus, the measurement of defocusing distance of the silicon wafer may be accomplished by measuring information about the change of the moiré fringes. Although such a method has a higher accuracy for detection, the system has a weak anti-interference capability and has a higher requirement for the environment.
In general, the currently reported method for detecting the focal plane has poor anti-interference capability and can't give attention to two or more things of the precision for detecting the focal plane and the efficiency. For the requirements of higher resolution for detecting the focal plane in the future projection photolithography machines, the present disclosure introduces a method for detecting the focal plane based on Grating Talbot Effect so as to meet the requirements of precisions and efficiency in the photolithography machines.